Firmware build 10.4.C.0.808 for the Xperia Z (C6606, C6616), which was certified back in April, is now starting to roll for T-Mobile US customers. According to the changelog, the new firmware adds the option to perform a master reset using the hardware keys. Firmware improvements include user interface improvements and the integration of Music Unlimited and Play Memories to the WALKMAN app and Album app respectively.
The update is rolling out from today, for more information click here. Another update with build number 10.4.C.0.814 was certified a fortnight ago, so there may be another update for T-Mobile Xperia Z owners soon.